Cattell referred to these 16 factors as primary factors, as opposed to the so-called "Big Five" factors which he considered global factors. All of the primary factors correlate with global factors and could therefore be considered subfactors within them.

Those individual differences that are most salient and socially relevant in people’s lives will eventually become encoded into their language; the more important such a difference, the more likely is it to become expressed as a single word.

This statement has become known as the Lexical Hypothesis.

Allport and Odbert had worked through two of the most comprehensive dictionaries of the English language available at the time, and extracted 18,000 personality-describing words. From this gigantic list they extracted 4500 personality-describing adjectives which they considered to describe observable and relatively permanent traits.

In 1946Raymond Cattell used the emerging technology of computers to analyse the Allport-Odbert list. He organized the list into 181 clusters and asked subjects to rate people whom they knew by the adjectives on the list. Using factor analysis Cattell generated twelve factors, and then included four factors which he thought ought to appear. The result was the hypothesis that individuals describe themselves and each other according to sixteen different, independent factors.

With these sixteen factors as a basis, Cattell went on to construct the 16PF Personality Questionnaire, which remains in use by universities and businesses for research, personnel selection and the like. Although subsequent research has failed to replicate his results, and it has been shown that he retained too many factors, the current 16PF takes these findings into account and is considered to be a very good test. In 1963, W.T. Norman replicated Cattell’s work and suggested that five factors would be sufficient.